Picking up very loud hum in my Corvette fretless... rewire?

[Vic]

I have a Corvette 4 string fretless, about 5 years old, with the MEC passive pickups. I'm picking up a terrible hum from the faders in the school auditorium lighting controls. My Peavey Foundation doesn't pick up any of the hum when I turn on both pickups.

I asked one of the salesmen in the local music store, he said that the Warwick pickups were not wired to act as humbuckers when the pickup mix knob is in the center position. He suggested either rotating one of the pickups 180 degress, or reversing the wiring of one of the pickups.

It was easier to reverse one of the pickups, so I reversed the bridge pickup. There was an improvement, but the hum is still bad.

I was thinking that reversing a pickup is not exactly the same as reversing the wiring of one pickup, as the pickups for each string are not in the same order, and therefore probably do not have the same amount of impedance and would not exactly cancel the phase hum.

Before I put the pickup back the way it was, and re-solder one of the pickup leads in reverse, I was wondering if anyone has had this problem, and whether reversing one of the pickup wires fixes it. Thanks.

Vic

 

[Escobon]

Re: Picking up very loud hum in my Corvette fretless... rewi

I was thinking that reversing a pickup is not exactly the same as reversing the wiring of one pickup, as the pickups for each string are not in the same order, and therefore probably do not have the same amount of impedance and would not exactly cancel the phase hum.

Vic

As an retired instrument repairman, I'll say that most often, hum problems are produced by lack of shielding, bad cables, or bad ground connections. Proximity to active magnetic fields (like being close to a power transformer) can induce hum into pickups. Reversing the polarity (phase) of a pickup (not recommended) is definitely different than rotating the position of a pickup. Effects of stray magnetic fields can be reduced by rotating pickup position, but it sounds to me that this problem may be native to the environment, and not necessarily the bass...

 

[Escobon]

Hum Problem...

Also, light dimmers are notorious for creating hum problems. They introduce spikes into the AC line that are picked and amplified by most amplifiers. Sometimes equipment with "Ground Lift" switches can help with this. Other times, using an AC circuit different than the circuit the dimmers are on can help. I wouldn't rewire. I'd consider improving shielding instead. Hope some of this helps,

Omar

 

[Vic]

Thanks,

I had already tried shielding, I purchased a shielding kit and lined the cavities under the pickups, as well as the box and lid that holds the pots, with heavy copper foil. That didn't have any effect.

The fact that my inexpensive Peavey Foundation doesn't pick up the hum when both pickups are on full, makes me wonder if maybe something is either broke or misconnected inside my Warwick.

There was a small blob of solder on one of the 3 leads coming out of the 1/4" plug in the Warwick. Only the center post and ground lead are connected to the pots, the third lead on the 1/4" jack is not connected to anything. Is that correct?


Vic

 

[Escobon]

Hummmmmmmm.....

So I can visualize better, verify for me that there are only two wires actually soldered to the contacts on the jack. The third contact has never been soldered to, and that there are no unsoldered wires close by. Also, check at the base of the jack (look for a nut inside the cavity that tightens it to the body) for a soldering lug that grounds the entire circuitry. This bass uses no batteries, right? If so, there's no need for the third contact.

The weak point on most instruments is the jack itself. Over time, the internal contacts lose their tension, and can produce hums, crackling, and complete signal loss. Does wiggling the guitar cable at the output jack produce any intermittence? This would indicate a loose jack.

Another trick that approaches voodoo is, with the guitar plugged into the amp, touch only the metal part (shield) of the jack. If this produces a change in the hum level (either louder or softer) then that would indicate an open connection either in the ground circuit or in the signal path.

Does the bass behave itself when plugged in elsewhere, like in another building? If not, the problem is definitely in the bass. Most likely the jack, but it's possible that elsewhere in the circuitry could be a bad connection, cold solder, loose wire, etc. Passive basses tend to have simple wiring, unless there are switches that change pickup phase, etc. Let me know what you find,

Omar

 

[odominguez]

Escobon you definitely beat me on this one. Those are great pieces of advice. Keep it up.